The present invention relates to an improvement in a hydraulic system for the boom cylinder of a working apparatus. The improved hydraulic system is suitable for use in, but not limited to, a construction machine such as a hydraulic excavator.
A construction machine such as a hydraulic excavator is commonly arranged to effect a desired work by activating a working apparatus which is operatively mounted on the body of the machine. Such a working apparatus generally has a large inertia since it is strongly made so as to endure heavy use and large load. For this reason, each time the working apparatus is actuated and stopped, it is oscillated or swung due to a large inertial force. The oscillation or swinging motion of the working apparatus affects working efficiency and operability such as ease of accurate positioning of the working apparatus. In addition, such oscillation may cause wear of moving parts to adversely affect the lifetime of the construction machine.
To suppress the above-described oscillation, for example, Japanese Patent Unexamined Publication Nos. 63-138024 and 63-40026 teach that a driving system is controlled, in accordance with the operating position of the control lever of a hydraulic excavator, so as to feed hydraulic fluid to a boom cylinder in a direction in which the oscillation of the boom is prevented. Either of the proposed oscillation-suppressing devices is arranged to execute the above control only when the control lever is placed at an operating position not beyond a predetermined reference position in order to prevent the progression of working from being hindered due to a discrepancy between the operational sensation of an operator and the operating speed of the working apparatus or the behavior of the working apparatus when stopping. That is, the above described control is not effected within an operational range substantially corresponding to the normal working of the machine.